Recently, the development of a wireless sensor network, in which sensor terminals formed of small wireless devices with sensors gather information such as ambient temperature, humidity, velocity, and images and transmit the information through multi-hop networking, has advanced.
It is generally assumed that the sensor terminals may be used outdoors, for example, at farms or in the city, and the power consumed by the sensor terminals needs to be reduced. Some methods for reducing the power consumption of the sensor terminals include reducing the power consumed by the devices in the sensor terminals or providing a sleep mode. However, in order to further reduce the power consumption, adaptive array antennas used for, for example, cellular systems may be introduced.
An adaptive array antenna in such a cellular system includes a plurality of antenna elements installed in a base station, and the beam directivity of the antenna to a terminal is increased while the directivity to interference sources is reduced to “nulls” (portions in which the sensitivity in the directivity pattern is minimized) by controlling the array weights (amplitudes and phases) of the antenna elements during signal communication. With this, signal transmission quality of the system may be increased.
Technologies for improving communication performance and reducing power consumption in wireless sensor networks by controlling the directivity of antennas have been developed to date (for example, see Japanese Unexamined Patent Application Publication No. 2004-289328).
According to a known technology, a transmitter first transmits a Request-to-Send (RTS) signal at a low transmission rate using a non-directional antenna, and a receiver receives the RTS signal using a non-directional antenna. Next, the receiver transmits a Clear-to-Send (CTS) signal to the transmitter at a low transmission rate, and switches from the non-directional antenna to a directional antenna. The transmitter that has received the CTS signal transmits data packets at a high data rate using the non-directional antenna without changing antennas. The receiver receives the data using the directional antenna.
According to the known technology, the receiver increases the directivity of the antenna to the transmitter after the RTS signal and the CTS signal are exchanged. With this, data may be transmitted at a high rate. However, this technology is intended for one-to-one communication between the receiver and the transmitter, and the case where a new transmitter starts communication with the receiver during the one-to-one communication is not considered. Therefore, the receiver may not receive RTS signals from other new transmitters and the like during the one-to-one communication using the directional antenna, and may not form a network.